1. Field of the Invention
The present invention relates to a battery remaining capacity measuring apparatus for measuring a remaining capacity of batteries used for an electric automobile or the like.
2. Description of Related Art
In general, an electric automobile is operated by a battery such as a lead storage battery or a nickel oxygen storage battery or the like, and therefore, accurateness is required for estimating a remaining capacity of such a battery. There are following methods for measuring the remaining capacity of a battery of this type.
(1) A current integrating system for measuring and integrating a current volume actually supplied from a battery to a load side and then estimating a current remaining capacity of the battery from a current integrated value obtained by this integration and a current volume at the time of a charging.
(2) A voltage detecting system for estimating the remaining capacity of a battery by measuring a terminal voltage at the time when a discharge current flowed from a terminal of the battery to a load side.
According to a battery remaining capacity measuring apparatus described in Japanese Patent Application Laid-open Publication No. 8-62310 which employs the voltage detecting system of (2), for example, a terminal voltage of a battery and a current flowing to a load are read by sampling at every 1 ms, for example. Then, the data read during a lapse of 100 ms period, for example, are collected and averaged for the voltage and current respectively. An averaged voltage value and an averaged current value are collected by 100 for each. Thus, based on the collected 100 data for average voltage values and average current values, an approximate straight line is obtained based on the method of least squares.
Next, based on the approximate straight line obtained, a voltage value corresponding to a predetermined discharge current is calculated, a remaining capacity of the battery corresponding to this voltage value is calculated, and the calculated remaining capacity of the battery is displayed.
In the case where an electric automobile is operated by a battery unit structured by a plurality of batteries connected in series, a total voltage and a total current of the whole batteries are measured respectively, and a remaining capacity of the batteries as a whole is calculated based on these measured total voltage and total current.
However, there is a case where the remaining capacity of a specific battery is lowered due to deterioration of this battery or other reason among the plurality of batteries. In such a case, as the calculated remaining capacity of the whole batteries shows an average remaining capacity of each battery, this average value shows a larger value than the noted minimum remaining capacity value of this specific battery. In other words, as it is not possible to obtain an accurate remaining capacity of the batteries, there has been a risk of an occurrence of an excess discharging of a battery due to a reduction of the remaining capacity of a specific battery.